[DSE][NFC] Rename Later->Killing, Earlier->Dead

First (and biggest) change is to use "Killing/Dead" in place of "Later/Earlier" base for names in DSE. For example, [Maybe]DeadLoc - is a location killed by KillingI instruction. I believe such names are more descriptive and easy to understand than current ones.

Second, there are inconsistencies in naming where different names are used for the same thing. Fixed that too.

Third, reordered parameters of isPartialOverwrite, tryToMergePartialOverlappingStores, isOverwrite to make them consistent between each other. This greatly reduces potential mistakes.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D106947